As for PcomA asymmetry, only the PA1 group exhibited a significant carotid flow and diameter difference, but to a lesser extent of 25%. A significantly higher carotid flow with an F-PCA compared with the symmetric group was only found after the exclusion of coincident A1 asymmetry. When A1 asymmetry occurred simultaneously with PcomA asymmetry (23 cases), a non-dominant A1 (19 cases) was more frequently on the same side of the dominant PcomA. A PcomA usually serves as a conduit connecting the anterior and posterior circulation to provide a collateral pathway when there is proximal vessel comprise or there is an incomplete COW. The average net flow of a PcomA is usually low, and in an anterior to posterior direction, and accounts for about 5% of the ipsilateral carotid flow [31]. Considering all the aforementioned findings, it is reasonable that PcomA asymmetry has little influence on carotid flow changes.